I was wondering this myself so I did a quick google search. Vientiane at 47th and Baltimore popped up in a random user review and so did Ong’s at 11th and Race. I can't vouch for these places at all (I hadn't even heard of them before) but it might be a good starting point to investigate further.

Ong's is no more. I have not been able to find vegan pho in Chinatown (or anywhere else in the city for that matter). I was super psyched when we got a Loving Hut, since all the LHs I've been to have pho, but ours doesn't. We should ask then to start offering it!